When to replace old drivers?

Well, I'm not familiar with your speaker cabinets, but any time yo start a DIY project like this you need to be prepared to go down unintentede rabbit holes :)

DeKay, I think I was the one who got that pair--great price, and when I saw how few of them are left out there... Good insurance. Yes, the LTA is a fantastic amp; although the ZOTL10 is listed as "power amp" it has a line stage an works just fine without a pre: more than enough for the Abbys. Thanks, everyone, for helping!

George:

Good/glad you snagged the EN's as I figured that my "(now)" may have been too subtle.

Have not seen the original Fostex Sigma and/or the later Sigma (168?) that have been discussed for use in the Abby's) on sale for years.

DeKay